1. Field of the Invention
This invention relates to telephone subscriber line interface circuits (SLIC's) for telecommunication transmission, data communication and telephone switching system applications. This invention also relates to the management of power within subscriber line interface circuits.
2. Description of the Relevant Art
In earlier years of telephony, transformer based circuits were used to interface the telephone subscriber line to a telephone exchange. With the advent of improved cost and performance parameters of integrated circuit technology, solid state subscriber line interface circuits (SLIC's) were developed and are currently in wide use in the telecommunications industry. These SLIC circuits allow for reduced equipment cost, reduced power dissipation, and reduced space consumption in comparison to their hybrid transformer predecessors.
Subscriber line interface circuits, which serve to connect the subscriber to digital switching equipment within the telephone exchange, are designed to perform a variety of functions. These functions include sourcing DC power to the subscriber loop while terminating the with the proper AC impedance, interfacing the two-wire loop with four-wire transmission equipment, rejecting longitudinal signals on the subscriber loop, and detecting the off-hook condition of subscriber equipment. A subscriber line interface circuit typically includes a current detector which detects the subscriber going off-hook during ringing (ring-trip). In particular, when the subscriber is being called, AC ringing signals are applied to the subscriber loop to operate the telephone ringer. When the subscriber goes off-hook, the resulting loop current is detected to interrupt the ringing signals on the loop. The transmission of alternating current voice signals is thereafter accommodated between the subscriber and telephone exchange through the SLIC circuitry.
FIG. 1 is a schematic diagram illustrating basic components within the receive output stage 10 of a typical subscriber line interface circuit 11 that is fabricated on an integrated circuit. The receive output stage 10 comprises a pair of operational amplifiers 12 and 14 configured as differential transconductance amplifiers. Such transconductance amplifiers generate an output current which is linearly dependent upon the differential input voltages applied thereto. The transconductance amplifiers typically provide a current gain of approximately 500. This current gain can be attained, for example, by selecting the resistance of a resistor 16 at 20 ohms and the resistance of a resistor 18 at 10 k ohms.
The receive output stage 10 is an integral portion of the SLIC circuitry known as the two-wire to four-wire converter. Other portions (not shown) of the two-wire to four-wire converter include a transmit amplifier and balance circuitry. The two-wire to four-wire converter inherently rejects longitudinal (common-mode) noise induced onto the subscriber loop from close proximity telephone and power cables. Additional suppression circuitry ensures that high-level longitudinal signals do not exceed the operating voltage range of the SLIC amplifiers.
The operational amplifier 12 drives a telephone subscriber loop tip line 15 through resistor 16 and the operational amplifier 14 drives a telephone subscriber loop ring line 17 through resistor 32. A telephone 20 is shown connected to the tip and ring lines 15 and 17. The circuit is powered from ground to a battery voltage of -50 volts and operates around a predetermined quiescent point within the range defined by these voltage rails.
An input and monitor circuit 25 is shown coupled to the receive output stage 10. The input and monitor circuit 25 receives voice signals through a system interface and correspondingly drives the receive output stage 10. The input and monitor circuit 25 provides a variety of other functions, including amplification of transmit signals provided from telephone 20 through lines 26 and 27. Control circuity is further incorporated within the input and monitor circuit 25 to support other so-called BORSCHT functions (battery feed, overvoltage protection, ringing, supervision, coding, hybrid, and test) of the SLIC. Implementations of such SLIC circuity are described in detail in the literature of the known prior art, such as in the IEEE Journal of Solid State Circuits, Vol. SC-16, No. 4, August 1981, pp. 261-276.
Industry standards specify that the DC current flowing through the subscriber loop when the telephone 20 is off-hook be nominally 30-40 mA. This DC current provides power to the telephone circuitry such as a digital keypad. The impedance of the subscriber loop depends upon the particular telephone connected to the loop as well as the transmission length of the loop. Typical values of loop resistance range between 0 to 2000 ohms. The SLIC must therefore be designed to provide the nominal DC current (I.sub.DC) for this range of loop impedances while still allowing transmission of the AC voice signals. One way of achieving this is to maintain node X at a DC voltage of approximately -5 volts while regulating the current flowing through resistor 32 such that the loop current equals the nominal current of, say, 40 mA. Such a system is known as an unbalanced system. It is noted that the input and monitor circuit 25 controls the operational amplifiers 12 and 14 to maintain the desired DC operating voltage and current. Depending upon the loop resistance, the DC voltage at node Y will be established at some operating point between -5 volts and the negative voltage rail of -50 volts. As represented by the waveforms at the tip and ring leads, a differential AC signal may be imposed upon the DC operating current to transmit voice signals.
The power dissipated by the transistor drivers of operational amplifier 14 in this situation depends upon the loop impedance. The power dissipated by the transistor drivers of operational amplifier 12 is, on the other hand, relatively constant and low since a voltage at node X is fixed at only approximately -5 volts. Take for example the case where the loop impedance is 1000 ohms. If resistors 16 and 32 are each selected at 20 ohms and node X is regulated at a DC voltage of -5 volts, then a DC voltage of -45 volts is established at node Y when the nominal current of 40 mA flows through the loop. On the other hand, if the loop impedance is only 300 ohms when the nominal 40 mA current flows through loop, a voltage of -17 volts is established at node Y. The voltage drop across each of resistors 16 and 32 is approximately 0.8 volts in either case. Thus, the power dissipated by the transistor drivers of operational amplifier 14 can be approximated by the voltage drop between the output line of the amplifier and the -50 volt supply. For the case where the loop resistance is 1000 ohms, the approximate power dissipation is (50-45.8 V).times.(40 mA)=168 mW. On the other hand, the power dissipated by the transistor drivers of operational amplifier 14 for the case where the loop resistance is 300 ohms is approximately (50-7.8 V).times.(40 mA)=1.29 W.
The power dissipated by the transistor drivers of operational amplifier 12 can be approximated by the voltage drop between the output line of the amplifier and ground. Since node X is maintained at a constant voltage, the power dissipated is (4.2-0 V).times.(40 mA)=168 mW regardless of the loop impedance.
From the foregoing, it is evident that the power dissipated by the driver transistors of operational amplifier 14 is dependent upon the loop impedance. The amount of power dissipated increases dramatically as the loop impedance decreases. Since the SLIC circuit must accommodate the broad range of possible loop impedances, the pull-down driver transistor within the output stage of operational amplifier 14 must sink a relatively large current when the loop impedance is low. This therefore requires that the driver transistor be relatively large and further results in increased heat generation within the integrated circuit when the loop impedance is low. This poses problems with the reliability of the integrated circuit and generally results in increased fabrication and packaging costs.